Near-focus intensity evolution of spatiotemporal optical vortex pulses and consequences for their nonlinear upconversion

Abstract

Spatiotemporal optical vortex (STOV) laser pulses are attracting attention for their puzzling transverse orbital angular momentum (OAM) and propagation properties, which could determine their future applications. While the characteristic reshaping of their space-time envelope upon diffraction from the focal plane to the far-field is now well-established, the evolution of their intensity as they propagate has not been studied quantitatively, despite growing interest for their use in nonlinear optics and strong-field physics. Here, we explore experimentally and analytically the focusing dynamics of STOVs produced by a pulse shaper, and evidence a substantial longitudinal shift between the coordinate at which a circularly-symmetric STOV forms and that at which the pulse reaches its peak intensity. To illustrate some practical consequences for applications, we experimentally demonstrate how the yield of second-harmonic generation can consequently be maximized several Rayleigh lengths beyond the plane where the circular STOV really occurs. These results signal careful characterization of spatiotemporal pulses as a requisite for nonlinear optical experiments, and pave the way for finer control of complex laser drivers for light-matter interactions.